How Earthquake Magnitude Is Actually Measured

The number behind the headline

Hours after any significant earthquake, one decimal number travels around the world: magnitude 6.8, magnitude 7.4. That number is not a score for destruction and it is not measured at your house. It is an estimate of the energy released at the earthquake's source, deep underground, computed from recordings made by seismographs.

An earthquake has exactly one magnitude, wherever you happened to be standing. What changes from place to place is the shaking you feel and the damage it causes, which is a separate measurement called intensity. Confusing the two is the most common mistake in reading earthquake news, so this guide keeps them apart from the start, follows the number from 1935 to today, and ends with a practical way to read the next report.

The Richter scale: a California tool from 1935

In 1935 the seismologist Charles F. Richter at the California Institute of Technology introduced the first practical magnitude scale. He compared the largest wave a seismograph recorded with the instrument's distance from the earthquake, producing a single comparable number for events in southern California.

His definition assumed a specific instrument, the Wood-Anderson torsion seismograph, earthquakes no farther than about 600 kilometres, and the shallow faults typical of California's upper crust. Within those limits it worked beautifully, which is why the name stuck in popular language long after science itself moved on.

A family of scales, each with its own range

As seismograph networks spread across the world, researchers found that Richter's method only held for certain wave frequencies and distances. New scales extended the same idea: body-wave magnitude, written Mb, uses deep-travelling waves, while surface-wave magnitude, Ms, uses the long rolling waves that travel along the surface.

Each scale is valid for a particular type of signal, and within its valid range each one was designed to agree with Richter's original numbers. The groundwork had been laid in 1931, when the Japanese seismologist Kiyoo Wadati showed that wave amplitudes fade with distance at a predictable rate, which made distance-corrected comparisons possible.

That calibration is why a moderate earthquake often carries the same value on three different scales, while very small or very large events expose their limits. When you see a magnitude reported without a letter code, it is usually one of this family, chosen automatically from whatever waves were clearest.

Moment magnitude: measuring the fault itself

In the 1970s the seismologists Hiroo Kanamori and Thomas C. Hanks defined a scale based not on the height of one wave but on the physics of the fault. The seismic moment combines three things: the rigidity of the rock, the area of the fault that slipped, and how far it moved. More slip, a bigger rupture area or stronger rock all mean a bigger earthquake.

Seismologists estimate this moment from the full shape of the recorded waves, then convert it with a fixed formula into moment magnitude, written Mw. The formula was calibrated so that moderate earthquakes get roughly the same number they would have had on the Richter scale, which keeps a century of records comparable.

What one extra unit actually means

Magnitude scales are logarithmic, so equal steps do not mean equal jumps. Each additional whole number means the seismograph trace is ten times taller, and the energy released grows by a factor of about 32.

Magnitude 5 to 6: ten times the wave amplitude, about 32 times the energy.
Magnitude 5 to 7: one hundred times the amplitude, about 1,000 times the energy.
The standard energy relation is roughly log E = 1.5 Mw + 4.8 (E in joules); the 1.5 is why each step means 10^1.5, about 32 times, more energy.
This is why a magnitude 7 headline describes a catastrophe while a magnitude 5 nearby may pass with broken dishes.

Why very large earthquakes broke the old scales

Amplitude-based scales suffer from saturation: above a certain size, the biggest waves stop growing in proportion to the energy, so the scale underestimates the event. On Richter's original scale, earthquakes larger than about 6.5 were mischaracterized and clustered near that ceiling.

Moment magnitude avoids saturation because it works from the total energy and the fault's geometry rather than a single peak. The largest earthquake ever recorded instrumentally, in Chile in 1960, is estimated at about Mw 9.4 to 9.6, a value no amplitude scale could have produced reliably.

Why early reports often disagree

The first magnitude you see is a fast estimate, computed within minutes from whichever stations reported first, and different agencies may use different scales and different data. As more waveforms arrive and the full moment analysis is completed, the number is revised, sometimes by several tenths.

Small earthquakes rarely get a moment magnitude at all: the US Geological Survey, for example, computes Mw mainly for events above roughly magnitude 3.5, while local networks report small quakes on scales closer to Richter's original ML. Two different numbers for the same event usually mean two different methods, not a calculation error.

Magnitude versus intensity: two different questions

Magnitude answers how big the earthquake was at its source. Intensity answers how strongly the ground shook where you are, and it varies from neighbourhood to neighbourhood with distance, depth, soil and building quality.

The best known intensity scale is the Modified Mercalli scale, which runs from I to XII and is defined by observed effects: rattling doors and cracked plaster at the low end, collapsed buildings at the top. The United States uses it, Japan uses its own Shindo scale, and much of Europe uses the European Macroseismic Scale.

MMI I-II: felt by very few people, usually on upper floors.
MMI IV-V: felt indoors by many; some windows or dishes break.
MMI VI-VII: felt by everyone; furniture moves and weak structures are damaged.
MMI VIII and above: serious damage and partial collapse of ordinary buildings.

How to read the next earthquake report

When the next headline appears, four details turn a scary number into real understanding: which magnitude type is being quoted, how deep the earthquake was, how far it was from populated areas, and what intensities were actually observed on the ground.

A deep magnitude 6 far offshore can be a footnote, while a shallow magnitude 5 under a city can be a disaster. Expect the first numbers to change within hours, trust the revised moment magnitude over the first alert, and look for local intensity reports to understand what people actually felt.

Sources

FAQ

Is the Richter scale still used today?

Mostly no. News reports still say Richter out of habit, but seismological agencies report moment magnitude for significant events, keeping Richter-style ML only for small local earthquakes.

Why do agencies report different magnitudes for the same earthquake?

Early numbers come from different stations, scales and quick methods. Once the full waveform data is analysed, agencies converge on a moment magnitude and the earlier estimates are revised.

What magnitude can people actually feel?

Below roughly magnitude 3, shaking is usually too weak to notice, matching intensity I on the Modified Mercalli scale. From about magnitude 3 to 4, people indoors near the epicentre start to feel it.

Does a higher magnitude always mean more damage?

No. Damage depends on depth, distance, local ground and construction quality. A smaller shallow earthquake under a city can hurt more people than a much larger one deep under the ocean.

About the Author

Written by the doyouknow.app editorial team with AI-assisted research and drafting, and checked against the sources listed above. Spotted an error? Email hello@doyouknow.app and we will correct it.

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